The present invention relates to holography and more particularly to an improved method for mastering and replicating holograms.
Replication of holograms is normally carried out by preparing a master hologram of the desired prescription which is then copied into another holographic recording material using a contact process. The master is usually made using a classical two-beam holographic recording system comprising an object beam and a reference beam. However, the master could itself be a copy of another master. In the case of a transmission hologram the copying process is based on interfering the diffracted and zero order beams produced by master to form a grating within the copy hologram material. Subject to processing variations such as shrinkage the holographic pattern or grating formed in the copy should be identical to the one in the master. This procedure may be used in mass production roll-to-roll processes. The principles of holographic replication and industrial processes for the mass production of holograms are well documented in the literature.
The optical design benefits of diffractive optical elements (DOEs) are well known, including unique and efficient form factors and the ability to encode complex optical functions such as optical power and diffusion into thin layers. Bragg gratings (also commonly termed volume phase grating or holograms), which offer the highest diffraction efficiencies, have been widely used in devices such as Head Up Displays. An important class of Bragg grating devices is known as a Switchable Bragg Grating (SBG). An SBG is a diffractive device formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture. Typically, SBG devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates or substrates. Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass substrates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied.
SBGs may be used to provide transmission or reflection gratings for free space applications. SBGs may be implemented as waveguide devices in which the HPDLC forms either the waveguide core or an evanescently coupled layer in proximity to the waveguide. In one particular configuration to be referred to here as Substrate Guided Optics (SGO) the parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. SGOs are currently of interest in a range of display and sensor applications. Although much of the earlier work on HPDLC has been directed at reflection holograms transmission devices are proving to be much more versatile as optical system building blocks and tend to be much easier to fabricate.
Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices.
One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. Transmission SBGs may not be used at near-grazing incidence as the diffraction efficiency of any grating for P polarization falls to zero when the included angle between the incident and reflected light is small. A glass light guide in air will propagate light by total internal reflection if the internal incidence angle is greater than about 42 degrees. Thus waveguide transmission SBGs may be used if the internal incidence angles are in the range of 42 to about 70 degrees, in which case the light extracted from the light guide by the gratings will be predominantly p-polarized.
Normally SBGs diffract when no voltage is applied and are switching into their optically passive state when a voltage is application other times. However SBGs can be designed to operate in reverse mode such that they diffract when a voltage is applied and remain optically passive at all other times. Methods for fabricating reverse mode SBGs are disclosed in a U.S. Provisional Patent Application No. 61/573,066. with filing date 24 Aug. 2011 by the present inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND which is incorporated by reference herein in its entirety. The same reference also discloses how SBGs may be fabricated using flexible plastic substrates to provide the benefits of improved ruggedness, reduce weight and safety in near eye applications.
The present invention is motivated by the requirement to record SBGs of differing optical prescriptions for use in image transmitting waveguides currently being designed for Head Up Displays (HUDs) and Head Mounted Displays (HMDs). The holograms may configured as stacks U.S. Pat. No. 8,233,204 entitled OPTICAL DISPLAYS U.S. patent application Ser. No. 13/844,456 entitled WIDE FIELD OF VIEW COLOR DISPLAY; or tessellated in single layers as disclosed in U.S. patent application Ser. No. 13/869,866 entitled APERTURE SAMPLING FOR DUAL AXIS SAMPLING. In such applications the holograms are used to tile a field of view (FOV) space and/or increase the size of the exit pupil. For large FOV full colour displays the number of holographic prescriptions can be high as the FOV of a holographic element is limited by diffraction efficiency angular bandwidth. Since the cost of fabricating masters using conventional holographic interferometry or ruling processes is currently very high this can make the manufacture of large FOV displays very expensive. Exemplary holographic masters and replicas thereof) are provided by companies such as Holographix Inc. (MA). Typically, masters are surface relief components fabricated using holographic, binary grating etching or mechanical ruling processes. Desirably, a mastering and replication process for large FOV holographic waveguides should provide a range of optical prescriptions spanning the required FOV space using a minimal number of master components. Ideally this should be accomplished with just one master. Applications such as HMDs and HUDs typically demand tight control of the diffraction efficiency and geometrical optical characteristics of the replicated holograms. In particular there is a need for precise control of the intensities of the diffracted and zero order beams. Currently available holographic mastering process suffer from the problem that the relative intensities of the diffracted and zero orders cannot be controlled to better than ±5%. As disclosed in a co-pending patent application PCT/GB2013/000273 the inventors have discovered that a perfect copy can be made if the master hologram is “over-modulated” by a small amount. Over-modulation in this context means that the refractive index modulation of the hologram is a little above that required to achieve the desired beam ratio. The next step is to separately attenuate the master beams to bring them to the desired ratio. Typically we require 50/50 or 1:1. However, the inventors have found that making a perfect master with the appropriate level of over-modulation, which is typically 5-10%, is very difficult in practice. To the best of the inventors' knowledge the required levels of index modulation control have not been achieved using conventional holographic recording processes using currently available holographic recording materials such as photopolymers and Photo Thermo Refractive (PTR) materials. Desirably a holographic mastering process should include methods for controlling the hologram modulation.
There is requirement for an efficient and cost-effective method for replicating holograms with a multiplicity of holographic prescriptions from a single master.